1. Field of the Invention
The present invention relates in general to valve operating mechanisms of an internal combustion engine, and more particularly to valve operating mechanisms for use in an internal combustion engine which has at least two intake or exhaust valves for each cylinder. More specifically, the present invention is concerned with valve operating mechanisms of a type which has a plurality of cams for actuating the valves, the cams being automatically switched in accordance with the operation condition of the engine.
2. Description of the Prior Art
As is known, in internal combustion engines, there is a type in which, for improving the output performance of the engine, two intake valves are provided for each cylinder. One of the engines of this type is shown in Automotive Journal named "Automotive Technique" Volume 45 No. 8 which was issued in 1991 from "Automotive Technology Society". In this prior art engine, under lower speed operation necessitating only a small amount of intake air, one of the two intake valves is kept closed to assuredly create a swirl in the associated combustion chamber. With this, stable combustion is obtained in the combustion chamber under such lower speed operation.
In order to clarify the task of the present invention, a valve operating mechanism for the above-mentioned prior art engine will be outlined with reference to FIG. 10 of the accompanying drawings.
As is seen from the drawing, the valve operating mechanism generally comprises a cam shaft 73 having two cams 71 and 72 mounted thereon and a hydraulically actuated rocker arm unit 77. The profile of the cam 72 is so formed as to induce a smaller valve lift. The rocker arm unit 77 includes a main rocker arm 75, a sub-rocker arm 76 and a hydraulic piston 74. Upon higher speed operation of the engine, a hydraulic pressure is applied to the piston 74 to move the same rightward in the drawing thereby uniting the two rocker arms 75 and 76. Thus, thereafter, the two associated intake valves (not shown) are forced to operate in accordance with the rotation of the cam 71. While, under lower speed operation of the engine, the piston 74 keeps the illustrated position permitting the two rocker arms 75 and 76 to operate independently in accordance with rotation of the respective cams 71 and 72. Thus, under this lower speed operation of the engine, the two intake valves are operated in accordance with the cams 71 and 72 respectively. In this case, because the cam 72 is so formed as to induce a smaller valve lift as mentioned hereinabove, one intake valve controlled by the cam 72 is kept substantially closed, so that a swirl is produced in the combustion chamber. That is, under such condition, the intake valve controlled by the cam 72 substantially takes a rest.
In general, a swirl produced in the combustion chamber is greatly affected by the flow speed of the intake air introduced into the combustion chamber through the intake ports. Thus, when, under lower speed and lower load operation necessitating only a small amount of intake air, the lift of the other intake valve (viz., the intake valve which is not subjected to the rest) is controlled relatively small, the flow speed of the intake air through this intake valve is increased and thus higher or stronger swirl is obtained in the combustion chamber.
However, if this measure is simply applied to the prior art valve mechanism of FIG. 10, that is, if the cam profile of the cam 71 is so formed as to induce a smaller valve lift, the air intake efficiency at the higher speed operation is greatly lowered due to the reduced lift of the two intake valves, which means lowering in output power of the engine. While, when, for the purpose of increasing the air intake efficiency at the higher speed operation, the cam 71 is so formed as to induce a larger cam lift, it becomes impossible to obtain a satisfied swirl at the lower speed operation of the engine.
That is, in the above-mentioned conventional valve operating mechanism, satisfied intake efficiency at the higher speed operation and satisfied swirl at the lower speed operation are not obtained at the same time.